CARBOHYDRATES

Carbohydrates are naturally occurring organic compounds containing carbon, hydrogen and oxygenwith oxygen in the ratio 1:2 as in water.  The general molelcular formula of carbohydrates is C_x(H₂O)_y  or  C_xH_2yO_y.  Carbohydrates are generally polydroxyl aldehydes, ketones or any compound that on hydrolysis yields any of the polyhydroxides.  They are synthesized in green plants by photosynthesis.

 

CLASSIFICATION OF CARBOHYDRATES

Carbohydrate

Simple sugar                             Complex sugar

(polysaccharides)

Monosaacharide           Disacharides                   e.g. starch cellulose

e.g. glucose                  e.g sucrose

 

EVALUATION

1. Define carbohydrates.
2. State one example each of simple sugar and complex sugar.

 

MONOSAACHARIDES

Monosaacharides are simple sugar with three to six carbon atoms per molecule.  The most common and important of these are the ones with six carbon atoms per molecule called the HEXOSES. They have the same molecular formula C₆H₁₂O₆ but different spatial arrangement.  Examples include glucose, fructose, galatose and mannose.

 

GLUCOSE (C₆H₁₂O₆)

Glucose, commonly known as grape sugar or dextrose, is present in grapes, in honey and the sap of plants.

Glucose is synthesized by green plants during photosynthesis.

 

PREPARATION:-  Glucose can be prepared in the laboratory by the acid hydrolysis of starch.

The starch is heated with dilute hydrochloric acid or tetraoxosulphate VI acid.  The acid functioning as hydrolyzing agent in the reaction.

(C₆H₁₀O₆)n      nH₂O                 nC₆H₁₂O₆

Starch          Dil H­₂SO₄             glucose

Physical properties

1. It is a white crystalline solid.
2. It is soluble in water.
3. It has sweet taste.

 

Chemical properties

1. It dehydrates to black residue of carbon when with conc. H₂SO₄

C₆H₁₂O₆                        6C + 6H₂O

2. It is a strong reducing agent. This is due to the presence of the –CHO group in the             molecule.
3. It is readily fermented to ethanol and carbon VI oxide by the enzyme (ZYMASE) in yeast.

 

TEST:  Add a few drops of Fehling’s solution to 5cm³ of glucose solution in a test tube.  A brick-            red precipitate is obtained on boiling.

 

GLUCOSE STRUCTURAL FORMULA:

H

C          =          O          the functional groups in the

H          C          H                      molecule are:

HO       C          O                      -CHO and OH

H          C          OH                   But -CHO determines the

H          C          OH                   chemical properties

OH       C          H

H

FRUCTOSE (C₆H₁₂O₆)

An isomer of glucose is fructose.   Fructose is a non-reducing sugar (it contains the >CO group)

                    FRUCTOSE STRUCTURAL FORMULA

H

 

H          C          OH       The functional groups in   the molecule are:

 

C    =                         -C=O and –OH

Chemical properties

HO       C          H

 

H          C          OH

 

H          C          OH

 

H          C          OH

 

 

EVALUATION

1. State two (2) physical properties of starch.
2. Give difference between glucose and starch.

 

DISACCHARIDES

Disaccharides are simple sugar containing two molecules of monosaccharide per molecules.  Their general molecular formula is C₁₂H₂₂O₁₁.  Some important disaccharides and the component monosaccharide are;

DISACCHARIDE                    COMPONENT MONOSACCHARIDES

Sucrose (cane sugar)                Glucose + fructose

Lactose (milk sugar)                 Glucose + galactose

Maltose (malt sugar)                 Glucose + glucose

 

SUCROSE

This is the common granulated sugar used at home to sweeten food.  It occurs naturally in plant and fruits e.g. pineapple, carrots sorghum, sap and sugar cane.

 

Preparation:  Sucrose is prepared industrially by;

1. Extraction of juice from sugar cane,
2. Concentration of the juice by evaporation,
3. Crystallization of the solid sugar from the concentrated juice and
4. Filtration of sugar crystals from the molasses.

 

PHYSICAL PROPERTIES:

1. It is a white crystalline solid.
2. It is dissolves readily in water.
3. It has sweet taste.

 

CHEMICAL PROPERTIES:

1. It chars by dehydration to a black residue of carbon on addition of Conc. H₂SO₄.

C₁₂H₂₂O₁₁                      12C  +  6H₂O

2. It undergoes hydrolysis to equal quantity of glucose and fructose when boiled with dil.

H₂SO₄ or HCL (aq)

3. It undergoes inversion under the influence of invertase in yeast

Sucrose                        invertase          Glucose + Fructose

4. It is non-reducing sugar.

 

TEST:

Glucose gives a red precipitate with freshly prepared Seliwanoff’s reagent.

 

USES

1. For sweetening foods and beverages.
2. For ethanol production by fermentation.

 

POLYSACCHARIDES

Polysaccharides are a group of complex carbohydrates composed of very long chains of monosaccharide linked together by condensation polymerization.

Monosaccharide            ^condensation           polysaccharides  +   n/₂  water

^Hydrolysis

Some important examples of polysaccharides are starch, glycogen, and cellulose.  The general molecular formula of polysaccharides is (C₆H₁₀O₅)n

 

EVALUATION

1. Name three disaccharides and the component monosaccharide.
2. Describe what would be observed on addition of Conc. H₂SO₄ to sucrose.

 

PROTEIN

Proteins are made up of polypetides chains.  Polypeptide chains are products of condensation polymerization of amino acid molecules.  The condensation polymerization of amino acid is the interaction between amino groups, -NH₂, and Carboxyl groups, -COOH, to form the polypeptide chain.

A polypeptide chain:

H          O                      H          O                     H

O

⁺H₃ N    C          C          [N         C          C]  +_nN C

O

R                        H        R                      H          R

Amino end                                dipeptide                      Carboxyl end

 

OCCURRENCE

Proteins are found in living cells where they perform different functions.  Examples are hemoglobin,  collagen and insulin, ribonucleic.  The structure have cross linkage between chains which are called disulphide bridges or disulphide cross linkages.

Properties

1. They are denatured easily at a temperature above 40⁰C by variation in PH and by certain organic solvents and chemical reagents.
2. Proteins are hydrolyzed to give amino acids by boiling with dilute acids (e.g. HCI) and alkalis or using enzymes.

 

Uses

1. They are used for building up of tissues and repairing of worn-out tissues in the body.
2. They are necessary for growth especially in infants.

 

Tests

1. Million’s test – A white ppt. is form when a few drops of Million’s reagent are added to some

egg-white in a test-tube.  The white ppt. turns brick red on heating (indicating the presence of proteins).

2. Biuret test: when few drops of dilute NaOH solution is added to some egg white solution in

a test tube, and 1% CUSO₄ solution is added drop by drop and the test solution is shaken

vigorously after each drop.  A violet colour is seen indicating a positive test for proteins.

3. Proteins form an intense yellow colour with a few drops of conc. HNO₃ acid.

 

EVALUATION

1. Explain a chemical test for proteins.
2. What is the product of hydrolysis of proteins?

 

AMINO-ACIDS

Amino acids are the basic structural units of proteins.

The functional groups in amino acids are amino group (-NH₂) and a carboxyl group (-COOH)

General structure

 

NH₂          α carbon

 

H        C         COOH

 

R

 

EVALUATION

1. Give the functional group of protein.
2. Draw the structural formula of a polypeptide.

α carbons are carbons in amino acids to which the functional groups are attached.

Examples – glycine, phenylalnine, alanine etc.  Amino acids are mainly in form of dipolar ions in neutral solution (zwitterions).

 

EVALUATION

1. Name the functional groups present in amino acids molecule.
2. What are proteins?
3. State two properties of protein.

 

WEEKEND ASSIGNMENT

1. One of the following is not a monosaccharide
2. glucose        B.  maltose       C.  fructose    D. maltose
3. The product obtained when sucrose is treated with dil. HCI are
4. glucose and galatose    B.  fructose and galatose    C.  glucose and fructose  D. glucose

 

3. Which of the following is a reducing sugar?
4. fructose       B.  glucose        C.  sucrose           D. ribose
5. Which one of these substances is not a polysaccaharides?
6. lactose         B.  starch          C.  cellulose         D. mannose
7. Few drops of conc HNO₃ is added to a solution and an intense yellow colour is obtained,

then the solution contained.

1. proteins       B.  carbohydrates         C.  fat     D. oil

 

THEORY

1. Explain a laboratory test for proteins.

2a.       What are carbohydrates?

1. Name the component monosaccaharide of

(i)         sucrose          (ii)     maltose.

 

See also

NATURAL AND SYNTHETIC POLYMERS

Fats and Oils As Higher Esters

ALKANOIC ACIDS

ALKANOLS

AROMATIC HYDROCARBONS